The present invention relates to a receiver device and communication system and, more particularly, to a receiver device and communication system that utilize interchannel interference in a communication system that comprises two or more channels or subchannels, that is, a single-carrier communication system comprising two orthogonal channels, a multicarrier communication system with filterbank modulation, DMT (Discrete Multi-Tone) modulation, FMT (Filtered Multi-Tone) modulation, or the like, or an OFDM or OFDM-CDMA multicarrier modulation communication system in which bandwidth is divided into a multiplicity of independent narrow subbands each of which is independently modulated. Further, when ‘channel’ appears hereinafter, same is intended to include such subchannels.
The bit error rate (BER) of a single carrier communication system comprising two orthogonal channels, or a multicarrier communication system or multicarrier modulation communication system permits additional improvement by utilizing a received signal that includes distortion arising from Inter Channel Interference (ICI). Inter Channel Interference is produced as a result of an unavoidable environment subject to orthogonal loss between channels that arises due to a system malfunction of the communication system or a time-varying channel. The Inter Channel Interference arises from the leakage of spectral energy or what is known as ‘interchannel crosstalk’.
The main advantage of the turbo receiver of the present invention is that the behavior of the ICI is treated as a probability variable of a Gaussian distribution with a zero mean value (a Gaussian approximation that is used in D. Froney. Jr, and M. V. Eyuboglu, “Combined Equalization and Coding Using Precoding”, IEEE Commun. Magazine, pp. 25-34, Dec. 1991, for example) and for which a finite-state discrete Markov process model is adopted. With this ICI model, a simple Gaussian approximation may be considered to be more realistic from the perspective the ICI quality. The turbo receiver of the present invention is based on a maximum posterior probability estimation algorithm. This turbo receiver is such that information that is derived from a first channel following nonlinear processing is used to examine the estimated maximum posterior probability and, similarly, information that is derived from the second channel is used to examine the estimated maximum probability of the first channel.
(a) Multicarrier Communication System
In a multicarrier communication system with filterbank modulation, DMT (Discrete Multi-Tone) modulation, FMT (Filtered Multi-Tone) modulation, or the like, that is, a multicarrier communication system that divides bandwidth into a plurality of subbands that are independent bandwidths and performs modulation according to transmission data for each subband independently, the selection of the filter set has traditionally been executed with the constraint that the Inter Symbol Interference (ISI) and Inter Channel Interference (ICI) be completely removed.
In the case of a virtual transmission channel with which there is no Doppler shift and no frequency offset between transmitter and receiver, that is, which does not induce signal distortion, this constraint guarantees error-free recovery of a transmission symbol by the receiver. However, a frequency offset that is produced in each channel as a result of inaccurate tuning of the oscillator or due to a Doppler shift brings about BER distortion due to spectral leakage or ICI.
The sole method of alleviating such BER deterioration is that of making the frequency offset as small as possible, more specifically, keeping same within 1% of the subcarrier frequency interval. However, this method necessitates an exact frequency offset estimation and there is the problem that, when a multicarrier signal mixed with noise is received and the noise level is large, the accuracy of the frequency offset estimation is impaired. In addition, this method does not work properly in the case of a high-speed phasing channel, that is, in the case of a high-speed phasing channel for which the Doppler shift is fixed with respect to the transmission symbol and varies with time.
As shown in FIG. 1, if the frequency offset (the frequency offset normalized according to the channel interval) a is zero (a=0), the transmission function of the first subchannel (gain/frequency characteristic) produces infinite decay at the center frequency f2 of the second subchannel (dotted line), as shown by the solid line in FIG. 1. Further, the transmission function of the second subchannel likewise produces infinite decay at the center frequency f1 of the first subchannel. That is, if the frequency offset a is zero, ICI is not produced between adjacent subchannels. In other words, if the frequency offset a is zero, the subchannels are orthogonal and the ICI does not exist at all. FIG. 2 shows the subchannel characteristic when the frequency offset a (≠0) exists in a DMT system. If the frequency offset a is not zero, each spectrum of the adjacent subchannel exhibits a non-zero reciprocal gain at the subchannel frequencies f1 and f2 of interest, which are specified as a21 and a12 in FIG. 2. That is, as shown in FIG. 2, when the frequency offset is not zero, ICI (crosstalk) is produced between subchannels.
(b) Single Carrier Communication System
With single-carrier modulation methods that are extensively used at present, the receiver must incorporate a Quadrature down converter (quadrature decoder). This must subject the RF signal or local oscillator output to a 90° phase shift. The phase shift of the RF signal is generally accompanied by a trade-off with a noise output gain and the phase shift of the RF signal is problematic in the case of a wide band signal of a high-speed data system. Hence, the phases of the I and Q signals (see FIG. 3) are desirably shifted. So too when there is an error in the 90° phase shift or mismatch of the Q quadrature signal amplitudes, the constellation of the signal that has been frequency down-converted (quadrature-decoded signal) is degraded, whereby the BER is increased. FIG. 3 represents an ideal case where the I and Q signal amplitudes are equal and the phases of the I and Q signals are orthogonal and FIG. 4 represents a case (phase error case) where the I and Q signal amplitudes are not equal or the phases of the I and Q signals are not orthogonal. In FIG. 4, because the phases of the I and Q signals (I′, Q′) are not orthogonal or the amplitudes thereof are not equal, an ICI Quadrature component is produced as indicated by the bold lines Iq and Qi.
The action of keeping the phase shift offset at 4 to 7° in order to retain the same quality as when there is no phase offset and the phase shift between the RF signals or I, Q signals is 90° was established through experimentation. As detailed earlier, conventional communication systems are faced by the problem that ICI occurs due to a frequency offset, phase error, or amplitude error, or the like, and the BER degrades as a result of this ICI.
(c) A General System Model
FIG. 5 is a multicarrier system in which the frequency offset shown in FIG. 2 exists or is a model of 2-channel ICI in a single-carrier system in which the orthogonal mismatch shown in FIG. 4 exists. 1 and 2 are transmitter devices of first and second subchannels ch.1 and ch.2 respectively; 3 and 4 are receiver devices of subchannels ch.1 and ch.2 respectively; 5 and 6 are the transmission channels of each subchannel; 7 and 8 are multipliers that multiply, for each of channel signals S1l*(t), S2l*(t), the crosstalk coefficients α12, α21; 9 and 10 are synthesizers that synthesize the crosstalk (ICI) from the other subchannels with the channel signal of the respective synthesizers' own channel; and 11 and 12 are noise synthesizers. As can be seen from FIG. 5, the first subchannel signal leaks to the second subchannel according to the coupling coefficient α12 and the second subchannel signal leaks to the first subchannel according to the coupling coefficient α21. As a result of the intersubchannel frequency orthogonal, the noise components, which are expressed as n1(t) and n2(t) are statistically independent (no correlation).
Assuming a case where binary information is transmitted via the first and second subchannels by means of multiple value modulation method 16 QAM with a symbol cycle T, each point of the 16 QAM constellation in FIG. 6 is expressed by a 4-bit symbol made up of common mode components i1, i2, and quadrature components q1, q2. Further, the four bits are interleaved for the sequence i1q1i2q2. The Quadrature components I and Q are each graycoded through allocation of the bits 01, 00, 10, and 11, which correspond to the levels 3d, d, −d, and −3d respectively.
The model in FIG. 5 is advantageous from the point of view of understanding the physical process that is the cause of ICI. If this model is used, the task is to be able to determine accurately the values of the received signals, the transmission information symbols, and so forth of each subchannel even when ICI is produced.
One possible method of alleviating ICI in a receiver device adopts the decision feedback equalizer (DFE) for ICI cancellation that is proposed by G. Cherubini, E. Eiefheriou, S. Olcer, and J. M. Cioffi, “Filter bank modulation techniques for very high speed digital subscriber line”, IEEE Commun. Magazine, vol. 38, pp. 98-104, May 2000.
Further, when the individual receiver device outputs are subject to a hard bit decision (hard decision) mode, there is barely any advantage in sharing information between the subchannels. The soft decision restricts the operational scope of the DFE according to the high SN ratio.
Even when the above approach is effective in a great number of real cases, minimizing the effects of ICI is essentially a quasi-optimum value. This is because, information on the transmission symbol is contained in the interference wave. The reliability of data transmission can be raised by means of optimal reception of a signal that is based on maximum posterior probability estimation.
An OFDM receiver that corrects errors in the phases and frequency of digital multiple carrier wave signals has been proposed as a first conventional technology (JP11-154926A). With this first conventional technology, in addition to demodulation FFT processing, the OFDM receiver performs FFT processing to evaluate its own noise component and prevents crosstalk (ICI) by performing correction processing so that the orthogonal remains between the carrier waves prior to the demodulation FFT processing on the basis of the evaluated own noise component.
Further, an OFDM receiver device that removes an intercarrier interference component that exists in the Fourier Transform output has been proposed as a second conventional technology (JP 2001-308820A). According to this second conventional technology, a filter coefficient is sequentially calculated by means of an adaptive algorithm to remove the carrier interference component from a frequency domain signal that constitutes the Fourier Transform output of the OFDM receiver and is established for an adaptive filter that is provided on the Fourier-Transform output side.
Moreover, the present inventors have proposed a receiver device that receives on each of two adjacent channels and closely examines the estimated probability of information on the second channel by means of the estimated probability of information on the first channel and, similarly, closely examines the estimated probability of information on the first channel by means of the estimated probability of information on the second channel (see WO 2004/023684). In this receiver device, the receiver portion that is provided on each channel calculates the difference between the probability that data received from the corresponding channel will be “0” and the probability that the data will be “1” as a soft decision target value in consideration of the degree of coupling between channels and adjusts and outputs its own soft decision target value by using the soft decision target value of the second channel that is inputted from the other receiver portion, whereby the “0” and “1” of the received data is judged on the basis of the adjusted soft decision target value.
However, the first and second conventional technologies do not demodulate the received signal so that the BER performance improves by using transmission symbol information that is contained in the ICI signal from the second channel.
Although the principle of the receiver device of the third convention technology is useful, the corresponding one bit of the adjacent subchannel is demodulated by using ICI but a plurality of bits are not demodulated by using ICI as one unit. For this reason, in communications in which a plurality of bits are taken as a single unit as per multi-value QAM modulation, there is the problem that reception and demodulation that uses information of a plurality of bits of the second channel is impossible.